Historically, the production of foodstuffs needed to support the continuing increase in global population has thus far been accomplished by increasing the total acreage under cultivation and by augmenting crop yields through the use of fertilizers. The expansion of cropland, however, essentially ceased years ago (Gilland, B., Endeavor 17:84-88 (1993)).
The continued increase in crop yields (a 250% increase in world cereal grain yield from 1950 to 1990) has been obtained by expanding and improving irrigation systems, by protecting crops from disease, insects, and the competition of weeds, and by the development of new varieties of plants (Gilland, B., Endeavor 17:84-88 (1993), see, Bohtool, B. B. et al., Plant Soil 141:1-11 (1992)). The application of increasing amounts of chemical fertilizers (nitrogen, phosphorus and potassium) was an important factor in this "Green Revolution."
Nitrogen is required in all protein, and is the most important of all plant nutrients. Nitrogenous fertilizers are derived from ammonia, which is produced from natural gas. Because chemical nitrogen fertilizers are produced from an energy source, its price is sensitive to fluctuations in world energy prices. Ammonia is often used as a fertilizer in more developed countries. The compound is hazardous, and must be stored as a gas and applied to fields under pressure. In much of the world, ammonia is converted into urea for use as a fertilizer. Although significant energy is required for this reaction, urea can be pelleted or powdered, and thus can be transported and handled more easily than ammonia.
Phosphorus is important for plant growth and seed formation. The phosphorus of chemical fertilizers is derived from "phosphate rock," which normally contains about 20-40% P.sub.2 O.sub.5. Unfortunately, P.sub.2 O.sub.5 is not water soluble, and thus the phosphate of P.sub.2 O.sub.5 is not readily accessible to plants. Thus, phosphate rock must generally be processed into a water soluble form (e.g., ammonium phosphate). Such processing requires the production, storage and handling of highly corrosive phosphoric acid intermediates.
Potassium is the least expensive of the common fertilizer. It is obtained from the mining of "potash," and does not require chemical processing. Potassium chloride (muriate of potash) comprises over 90% of the potash that is mined. The remaining potashes are chiefly potassium sulfate and nitrate.
Several factors indicate that chemical fertilizer production will not be able to support future increases in crop yield (Bohtool, B. B. et al., Plant Soil 141:1-11 (1992)). Substantial energy is needed to produce nitrogen fertilizers. It has been estimated that 1.3% of world energy consumption is needed to produce existing stores of nitrogen based fertilizers (Gilland, B., Endeavor 17:84-88 (1993)). As world population grows, the price and availability of energy is thus likely to have a significant impact on the production of nitrogen fertilizers. Irrigation and rain water "run-off" and leaching of applied fertilizers cause the eutrophication of lakes, rivers, and bays, and thus substantially contribute to water pollution. Such pollution will only be exacerbated by an even wider application of fertilizers.
Several approaches have been taken in the hope of improving the economics of fertilizer use and of lessening the adverse ecological effects of fertilizers. One approach concerns the use of "controlled release" formulations. Increased crop yields can be expected if the rate of nutrient supply is adapted to the physiological needs of the crop (Oertli, J. J., Fertil. Res. 1:103-123 (1980)). By providing a season's nutritional requirements in a single application, soil compaction is reduced, there is less mechanical damage to crops, and a savings in labor can be realized.
Premature dissolution of water-soluble fertilizers has been regulated by granulating the fertilizers and coating the granules with a diffusion barrier (Oertli, J. J., Fertil. Res. 1:103-123 (1980)). A variety of suitable coatings has been described. Osmocote, for example, is a co-polymer of dicyclopentadiene and an oil derived from soybean or linseed (Jung, J. et al., In: Die Landwirtschaftliche Versuchsstation Limbergerhof (1914-1964), Herausg. Badische & Soda-Fabrik, Ludwigshafen, Landwirtschaftliche abt., pp. 164-182 (1964)). When the granules take up water, the fertilizer salts dissolve, generating a high internal hydrostatic pressure that presses the nutrients out of the granule.
Molten sulfur has also been used as a coating material (Jarrell, W. M. et al., Soil Sci. Soc. Amer. J. 43602-605 (1979); (Jarrell, W. M. et al., Soil Sci. Soc. Amer. J. 431044-1050 (1979)). The granules may contain a microbicide to retard microbial growth, and a wax or petroleum-based sealant to retard dissolution. A conditioner (i.e., kaolinite, diatomaceous earth, or vermiculite) is added to the final granule in order to prevent granule self adhesion.
Controlled release has been accomplished by formulating the fertilizer with nutrients that are in the form of sparingly soluble compounds. Dissolution of the compound by the plant shifts the controlling equilibrium such that more of the compound dissolves. Metals, such as iron or magnesium have also been used to shift the equilibrium of dissolution, thereby altering the rate of release.
Various organic molecules (i.e., urea formaldehyde polymers, isobutylidenediureas, crotonylidenediureas, acetaldehyde-urea) have also been used to control nutrient release rates (Oertli, J. J., Fertil. Res. 1:103-123 (1980)).
Microbial fertilizers have been proposed as an alternative to chemical fertilizers (Giller K. E. et al., In: Nitrogen Fixation in Tropical Cropping Systems, Ocon:C.A.B. International (1991); Babu, C. R. et al., Proc. Indian Nat'l. Sci. Acad. B59:359-366 (1993)). Several genera of bacteria (collectively termed "rhizobia" are capable of fixing atmospheric nitrogen into nitrates or ammonia. The rhizobia form tubercles (or nodules) within the roots of plants. These nodules comprise a symbiotic relationship between the rhizobia and the plant, in which the rhizobia provides reduced nitrogen in return for other nutrients provided by the plant (Guar, Y. D., Proc. Indian Nat'l. Sci. Acad. B59:333-358 (1993)). Unfortunately, rhizobia are essentially unable to colonize certain economically important classes of plants (such as the cereals, grasses, fruit trees, etc.). Efforts to provide nitrogen to cereals by mixed cultivation with nodulated legumes has been largely unsuccessful (Guar, Y. D., Proc. Indian Nat'l. Sci. Acad. B59:333-358 (1993)). Moreover, the high cost of present microbial fertilizers renders their use economically unfeasible.
Stains of Actinomycetes and Bacillus have been isolated that can solubilize the phosphorus of rock phosphorus, and their use in providing phosphates to crops has been attempted (Banik, S. et al., Zentralblatt fur Bakteriologie: Abteilung 136:478-486 (1981); Banik, S. et al., Plant Soil 69:353-364 (1982); see also MBA, C. C., Exper. Manag. 18:257-261 (1994) and MBA, C.C., Exper. Manag. 18:263-269 (1994)).
The possibility of using nitrogen fixing bacteria as fertilizers has been complicated by the ecological constraints encountered in attempting to establish a non-native bacteria within an existing ecosystem (Bohtool, B. B. et al., Plant Soil 141:1-11 (1992)). These factors include the competitive ability of the nitrogen fixing bacteria, the magnitude of the naturally occurring soil genera, etc. (Babu, C. R. et al., Proc. Indian Nat'l. Sci. Acad. B59:359-366 (1993)). In addition, the presence and concentration of micronutrients (such as copper, iron, molybdenum, zinc, manganese and boron) affect the capacity of microbial fertilizers to colonize root nodules and mediate nitrogen fixation (Bhanavase, D. B. et al., J. Maharashtra Agric. Univ. 18:167-174 (1993)). A nitrogen-fixing Azotobacter species has been used to provide nitrogen to rice, wheat and maize (LauWong, M. M., Agric. Ecosys. Environ 19:145-153 (1987)). The trials demonstrated a statistically significant effect for rice, however, no significant effect was observed for wheat or maize (Lau-Wong, M. M., Agric. Ecosys. Environ 19:145-153 (1987)).
In view of the importance of an alternative to the use of chemical fertilizers, it would be desirable to have a biological fertilizer which would not damage the environment, and which could be employed in an economically feasible manner. The present invention provides such a fertilizer, as well as methods for its use.